In a semiconductor manufacturing process, a film forming process is carried out to form a SiN (silicon nitride) film on a substrate so as to cover a base film having a stepped portion formed by a hole, recess or the like in the substrate. The SiN film is required to be formed at a uniform film thickness throughout respective portions of the substrate. A film forming apparatus for forming a SiN film on a base film by Atomic Layer Deposition (ALD) is known in the art.
In a film forming process using such an ALD, a film forming gas is supplied to a surface of a substrate to form a thin layer which covers the whole surface of the substrate. The thin layer starts to grow to form a film. A time from when the film forming gas is supplied till when the thin layer is formed (namely, when the film starts to be formed) is called an incubation time. This incubation time depends on the type of a base film constituting the surface of the substrate on which a film is formed.
There may be a case where a stepped portion is formed by multiple types of base films stacked one above another, and these base films are exposed as a surface of a substrate. A method of forming a SiN film on the substrate having the base films thus formed is under consideration. However, there is a difference in incubation time between the base films as described above. This makes it difficult to enhance a film thickness uniformity of the SiN film in-plane of the substrate.